1. Field of the Invention
This invention relates to a process for producing mesophase pitch.
In accordance with this invention, mesophase pitch containing quinoline soluble mesophase may be produced in a short heat treatment time without conducting any special treatment such as extraction.
2. Description of the Prior Art
Most of commercially available carbon fibers are currently produced from polyacrylonitrile fibers. However, they are extremely expensive because the starting material polyacrylonitrile fibers are expensive and the yield of carbon fiber obtained is low. On the other hand, a process utilizing carbonaceous pitch as a starting material has particularly aroused great interest in recent years since the starting material is inexpensive and the yield of carbon fiber is high.
Carbon fiber obtained from an optically isotropic pitch is low both in strength and modulus. In order to obtain a high-strength, high-modulus carbon fiber, it is necessary to subject an isotropic pitch to heat treatment to form mesophase which is a kind of liquid crystal. The pitch containing such mesophase, especially the pitch containing 40% or more of mesophase suitable for the production of high-performance carbon fibers, is called the "mesophase pitch". A representative process for producing mesophase pitch is, as described in Japanese Patent Publication No. 1810/1979, that comprises heating an isotropic pitch at 400.degree. C. for 17 hours, thereby forming about 50% of mesophase. However, almost all mesophase obtained by this process is insoluble in quinoline and pyridine, and therefore the softening point of mesophase pitch was high and hence spinning was difficult.
On the contrary, mesophase pitch containing quinoline soluble mesophase has the advantage that the softening point is low and thus spinning is facilitated, and it has drawn attention as a suitable starting material for high-performance carbon fibers. Examples of the prior art relating to the production of mesophase pitch containing quinoline soluble mesophase are as follows:
In Japanese Patent Application Laid-open No. 160427/1979, a quinoline soluble mesophase is obtained by extracting an isotropic pitch with a solvent and heating the insolubles at 230.degree.-400.degree. C. This is referred to in the present specification as the neomesophase and the solvent insolubles before heating as the neomesophase former or NMF fraction.
Further, in a series of patents, there have been proposed the increase of the NMF fraction in quantity, the improvement of the method for separating the NMF fraction. (see Japanese Patent Application Laid-open Nos. 58287/1980, 130809/1980, 144087/1980, 2388/1981, 109807/1981, 167788/1981 and 2393/1982, respectively).
In Japanese Patent Application Laid-open No. 55625/1979, mesophase pitch containing pyridine soluble mesophase is obtained by heat treating an isotropic pitch at 380.degree.-430.degree. C. with stirring while passing an inert gas thereover. The heating time in this methods is 2-60 hours.
In Japanese Patent Application Laid-open No. 57881/1981, mesophase pitch containing pyridine soluble mesophase is obtained by subjecting a pitch to a physical operation such as extraction with a solvent.
Pitches used in these methods are mainly petroleum pitch and coal tar pitch. It is reported that when producing mesophase pitch containing 40% or more of mesophase from ethylene tar pitch (pitch starting from an ethylene bottom) having properties different from those of these pitches, nothing but that containing more than 60% of pyridine insolubles can be obtained [see S. Chwastiak, I. C. Lewis, Carbon, 16, 156-157, (1978)].
In Japanese Patent Application Laid-open No. 101915/1981, mesophase pitch containing pyridine soluble mesophase is obtained by heat treating a pitch precursor such as ethylene tar at 400.degree.-550.degree. C. under pressure and subsequently heat treating it under atmospheric pressure while passing an inert gas thereover. The heat treatment conditions are similar to those described in Japanese Patent Application Laid-open No. 55625/1979, and have a problem that the heating time is long.
In addition to these known techniques, a method which comprises reacting pitch with tetrahydroquinoline at about 400.degree. C., subsequently heat treating at about 500.degree. C. under reduced pressure for a short time to obtain an isotropic pitch called the premesophase, and obtaining a high-performance carbon fiber therefrom has been published (Nippon Keizai Shinbun dated Aug. 22, 1981). Furthermore, in Japanese Patent Application Laid-open No. 157679/1980, a high purity petroleum pitch is obtained by treating a petroleum residual oil with hydrogen and heat treating it under pressure.
As described above, none of the known techniques were free from problems, for example, a special treatment such as extraction was needed for producing mesophase pitch, the steps were complicated, the treating time was long.
It was previously discovered that when using a catalytically cracked bottom as a starting material and, without any pre-treatment, heat treating it at 430.degree.-550.degree. C. under reduced pressure or while passing an inert gas thereover until at least 40% of mesophase has been formed, mesophase pitch containing quinoline soluble mesophase may be obtained. This pitch may be easily spun to form a high-performance carbon fiber. The present inventors have filed application for patents related thereto (Japanese Patent Application Nos. 24899/1982 and 24900/1982.
However, when a heat treatment similar to that described in these patents was conducted on a pitch, obtained by removing low-boiling distillates from a starting material employed in the process of this invention described hereinbelow, for example, an ethylene bottom, the formed mesophase pitch had a high softening point and thus was not suitable for spinning.
Various kinds of starting material pitches, their properties, further treating conditions of these pitches have been studied, in pursuit of a process of producing high-performance mesophase pitch more advantageous for commercial production than the above-described known techniques. As a result, it has been discovered that by using a specific pitch and conducting a treatment in a high temperature region, which had not been conventionally employed under reduced pressure or while passing an inert gas thereover, a low softening point high-performance mesophase pitch containing quinoline soluble mesophase may be obtained without conducting any special treatment.